Aim for general template variables for the Dockerfile template rendering

[consideRatio]

I think both of these PRs are suggesting we pass a value to repo2docker via a dedicated flag that can override config, that can then influence how a Dockerfile template renders.

• Let FROM <base_image> in the Dockerfile template be configurable #909
• Let CMD <cmd> in the Dockerfile template be configurable #957

I'd love to see a more general pattern, where we support passing template variables of all kinds instead and not needing to add dedicated flags etc.